Gear

Technical Q&A with Lennard Zinn – Leg length; Mixing and matching

Dear Lennard,My right leg is half-an-inch shorter than my left. The difference appearsto come from a shorter right tibia. I get saddles sores on the right side.Should I shim or raise my right cleat? If so, how? I ride Look A-5 pedaland Sidi Titan blue cleats. Thanks for any input you might have.Rick Dear Rick,With a shorter tibia, a cleat shim of the same thickness as your leg-lengthdiscrepancy should work out pretty well. But it is not always so simpleas that. I get so many questions about leg-length discrepancies that inthe Bike Fit section in my new “Zinn’sCycling Primer,” I have devoted

By Lennard Zinn

Technical Q&A with Lennard Zinn - Leg length; Mixing and matching

Technical Q&A with Lennard Zinn – Leg length; Mixing and matching

Photo:

Dear Lennard,
My right leg is half-an-inch shorter than my left. The difference appearsto come from a shorter right tibia. I get saddles sores on the right side.Should I shim or raise my right cleat? If so, how? I ride Look A-5 pedaland Sidi Titan blue cleats. Thanks for any input you might have.
Rick

Dear Rick,
With a shorter tibia, a cleat shim of the same thickness as your leg-lengthdiscrepancy should work out pretty well. But it is not always so simpleas that. I get so many questions about leg-length discrepancies that inthe Bike Fit section in my new “Zinn’sCycling Primer,” I have devoted two entire “blocks” (in my book, these”blocks” are like a small chapter—there are 50 of them in the book) andparts of several others to this subject.

Over time, even with a simple tibial length discrepancy like you mayhave (femoral length discrepancies are much more problematic to deal with)treated with a cleat shim, riders often develop debilitating back painand other unpleasant symptoms. For instance, 1984 Olympic road championAlexi Grewal, a teammate of mine both in the Coors Classic and on our tradeteam, had a 3/8-inch leg length discrepancy – less than yours – also inthe lower leg (his was due to breaking his leg on the growth plate whenhe was 16, and when it came out of the cast, it was longer than the otherleg), and his back pain became almost unbearable at times.

This was during the era of toe clips and my own personal opinion isthat he benefited in the Olympics  – and the months leading up tothe Games – from the use of Shimano’s Dyna-Drive dropped pedals, whichlined up the ball of the foot with the center of rotation of the pedal.That way, his shimmed foot ended up with no more rocking torque (the rotationalforce required to keep the foot above the pedal spindle) than riders onCampy quill pedals. But when Shimano stopped making those pedals, I thinkhe became worse off.

Lately I have been directly emailing an electronic file of one of theblocks of my book on leg-length discrepancies to people who write in. ButI think it actually shortchanges them, because unless I send four or fivewhole blocks that talk about the problem and the photos as well, they donot have as much information to work with as the book will provide. I thinkthat any of you with questions about leg length differences will be bestserved by waiting until the book comes out next month, since there aremany ways of treating this problem. Not all of them may work for you, soit is best to have an overview of a number of methods.
Lennard

Mix and match
Dear Lennard,
Your articles and books over the years have been extremely informative.I currently have some questions regarding Shimano 9- and 10-speed compatibilityand hope you can help since I seem to get different answers depending whoI ask. I need new cranks but do not want to buy 9-speed cranks that willbe phased out and not compatible with future upgrades. The Shimano websiteoffers some help but seemingly contradicts itself:

6. Will the new cranks (FC-7800) work with my current9-speed set up? The new cranks are designed to be used with the new frontderailleur (FD-7800) and chain (CN-7800). The pickup pins and ramps ofthe new crank will work best with these new parts.

8. Can you run the new Dura-Ace chain (CN-7800) with current9-speed drivetrains? The new Dura-Ace chain is compatible only with thenew 10-speed Dura-Ace components.

So I should use the 10-speed chain… but it says it will not work withmy 9-speed cogs. An article I recently read showed that the Webcor teamis running the FC-7800 crank with the 9-speed drivetrain components anda SRAM chain. Are you aware of any compatibility issues between the Dura-Ace9 and 10-speed drive trains? Also, what is the difference between FSA’s9 and 10 speed cranks? Do they just use different chain rings or are thespiders actually different?
KK

Dear KK,
This is another subject about which I receive a ton of mail, so itis high time I posted one of these questions.

My experience has been that it makes little difference. I have beenbrazenly using Shimano 9-speed chainrings on my custom cranks with dozensof different customers over the years, regardless of whether they havea Shimano 8- or 9-speed drivetrain or a Campy 8-, 9- or 10-speed drivetrain,and we have experienced no problems. Indeed, Chris Horner has been doingfine on a Dura-Ace 10-speed crank with a SRAM 9-sped chain and cogset andDura-Ace 9-speed STI levers and derailleurs.

On one of my own bikes, I have a Dura-Ace 7800 (10-speed) crank witha Wippermann 9-speed chain, a Campy Record 9-speed front derailleur, andDura-Ace 7701 (9-speed) STI levers. It works fine.

Technical Q&A with Lennard Zinn - Leg length; Mixing and matching

Technical Q&A with Lennard Zinn – Leg length; Mixing and matching

Photo:

I constantly mix and match all kinds of different cranks and chainringson my four road bikes, two of which are Shimano 9-speed and two of whichare Campy 10-speed, and I have never had any problems. I recently startedmeasuring chainring thickness and spider tab thickness of some of the slewsof cranks I have sitting around here. Maybe I will finish sometime andtell you the results. So far, what I can say is that there is quite a variationin tab width from 3.4mm to 3.6mm, even within the same crank, yet I haveused all of them without problem on my own bikes.
Lennard

Answer from FSA:

FSA has recently changed the spacing of our road cranksetsin order to accommodate users of DA 10. Up until approx February 2004,we always used the same chainrings for both S-9 and C-10. We differentiatedthe spacing by making the spider tabs different widths.

Starting in February 2004 we started to ship our road cranksets usinga single standard spacing that we have found works with S-9/S-10 and C-10.Cranks with this new spacing have a yellow sticker on the box, which indicatesthe three-format compatibility. We continue to use the same chainringsas before. Spacing from the narrowest (S-10) to the widest (C-10) is amatter of 10ths of millimeters. While this variance is not immaterial,it was possible to find a common spacing that works well with all threedrivetrain formats.
Doug Stuart
FSA
 

Meanwhile, dear readers, the letters on tubeless tire topic continue topour in and I will put some of them up next week. I just needed a breakfrom the subject for a while. – Lennard


Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides “ Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.

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